Fatigue testing machine

ABSTRACT

THE FATIGUE TESTING MACHINE OF THIS INVENTION COMPRISES OPPOSED, RE-ENTRANT STACKS OF PIEZOELECTRIC ELEMENTS, A PORTION OF THE STACK BEING FORMED FROM RINGS OR ANNULI OF PIEZOELECTRIC MATERIAL, AND EACH STACK HAVING AN INNER CORE PORTION FORMED OF PIEZOELECTRIC DISCS OR PLATES. THE SPECIMEN BEING TESTED IS HELD BY THE FREE ENDS OF THE INNER CORES. THE STACKS ARE DRIVEN IN OPPOSITE DIRECTIONS SIMULTANEOUSLY, THAT IS 180* OUT OF PHASE, TO GET THE DESIRED PULLING AND COMPRESSION ON THE TEST SPECIMEN. ONE OF THE STACKS IS MOUNTED ON A PLATFORM WHICH IS SUPPORTED ON A VISCOUSLY DAMPED PISTON TO COMPENSATE THE APPARATUS AGAINST THE EFFECTS OF STATIC LOADING ON THE SPECIMEN.

Feb. 16, 1971 F. E. REED, JR

FATIGUE TESTING MACHINE Filed Oct. 15, 1968 6 SheetsSheet 1 8 2 4 x w/ a w 7 w M Q o MWV rmw a A H w o 5 H w w. unwn I w m Q E E F 4 FIG. I

31: Hq di ATTORNEYS Feb. 16, 1971 F. E. REED, JR

FATIGUE TESTING MACHINE Filed Oct. 15. 1968 6 Sheets-Sheet 2 97 INVENTOR FLOOD EVERETT REED, JR. BY

6 I JWT f ATTORNEYS F. E. REED, JR

Feb. 16, 1971 FATIGUE TESTING MACHINE 6 Sheets-Sheet 4.

Filed Oct, 15, 1968 INVENTOR.

FLOOD EVERETT REED, JR.

ATTORNEYS Filed Oct. 15, 1968 F. E. REED, JR

FATIGUE TESTING MACHINE 6 Sheets-Sheet 5 202 TOP I CORE 644:: 64fm STACK 204 272 f 240 TOP I02: |O2-n= 270 L I T 62 240' BOTTOM 204' i BOTTOM CORE 64 :2 STACK L 60 INVENTOR.

FLOOD EVERETT REED, JR.

ATTORNEYS Feb. 16, 1971 F. E. REED, JR

FATIGUE TESTING MACHINE 6 Sheets-Sheet 6 Filed Oct. 15, 1968 O O O 5 4 3 9.60 9 z E zorrum uwo I000 IIOO I200 I30 FREQUENCY IN C PS FIG. I l INVENTORL FLOOD EVERETT REED, JR

ATTORNEYS United States Patent O 3,563,086 FATIGUE TESTING MACHINE Flood Everett Reed, Jr., Littleton, Mass., assignor to Littleton Research and Engineering Corporation, Littleton, Mass., a corporation of Massachusetts Filed Oct. 15, 1968, Ser. No. 767,801 Int. Cl. G01n 3/32 US. Cl. 73-92 Claims ABSTRACT OF THE DISCLOSURE The fatigue testing machine of this invention comprises opposed, re-entrant stacks of piezoelectric elements, a portion of the stack being formed from rings or annuli of piezoelectric material, and each stack having an inner core portion formed of piezoelectric discs or plates. The specimen being tested is held by the free ends of the inner cores. The stacks are driven in opposite directions simultaneously, that is, 180 out of phase, to get the desired pulling and compression on the test specimen, One of the stacks is mounted on a platform which is supported on a viscously damped piston to compensate the apparatus against the effects of static loading on the specimen.

BACKGROUND OF INVENTION The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).

The concept of utilizing piezoelectric driving elements for fatigue testing is not new. However, in the prior art, the piezoelectric elements used either singly or in a stack do not provide the excursion necessary for the fatigue testing of metals, since piezoelectric materials in and of themselves do not have a large excursion per unit length of material. It is for this reason that some of the prior inventions utilize a cone or horn-shaped element whereby the relatively small excursion of the entire face of the piezoelectric transducer is transformed through resonance by the horn member into the relatively small cross-sectional area but high excursion and velocity portion of the horn.

Also, in the prior art, no means are provided to elim inate the effect of static loading. However, in fatigue testing, the problem of static loading can seriously effect the accuracy of the results. One of the most serious causes of unwanted static loading is the thermal expansion of the sructural members which comprise the fatigue testing machine, and also the thermal expansion of the test element itself. Where piezoelectric elements are used, naturally they tend to heat because of the energy being dissipated therein and thereby, and they in turn will tend to expand under the influence of temperature. All of these changes in physical dimension preferably need to be compensated for, and the instant invention provides an apparatus having such compensation.

SUMMARY OF INVENTION The invention therefore provides an apparatus for fatigue testing metal both in tension and compression using piezoelectric materials as the direct force applying means, and includes a compensating mechanism to eliminate the effect of static loading which may be caused, for

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example, by thermal expansion of the structural members of the apparatus, the test member itself, and the piezoelectric driving elements. In addition, because of the structure of the device, tests on the specimen may be made at frequencies below the resonant frequency, the apparatus having approximately a linear response over the operating range of test frequencies to be used.

Therefore, among the several objects and advantages of the invention may be noted the provision of fatigue testing apparatus which uses opposed piezoelectric element structures as the force-applying means to fatigue test a specimen mounted between the structures; apparatus of the last-named class in which a large driving force and relatively large physical excursion of a stack or assembly of piezoelectric elements on either side of the test specimen are obtained without unduly increasing the overall size of the apparatus; the provision of apparatus of the above class in which the piezoelectric driving elements are grouped in two re-entrant stacks; the provision of apparatus of any of the above classes in which means are provided for compensating for the effects of static loading on the test specimen; the provision of apparatus of the last-named class in which the means provided is insensitive to physical motions taking place over a long period of time, but is relatively sensitive to motions occurring at a relatively high frequency; and the provision of testing apparatus of any of the above kinds which is easy to make and relatively simple and accurate to use. Other objects and advantages will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the elements and combinations of elements, features of construction, arrangements of parts and manipulation of the apparatus all of which will be exemplified in the structures hereinafter set forth, and the scope of the application of which will be indicated in the appended claims.

Referring now to the drawings, in which is illustrated one of the several possible embodiments of the invention:

FIG. 1 is an elevation of the embodiment, showing the general assembly and relationship of the various units thereof, a portion of the base being broken away to reveal other parts more clearly;

FIG. 2 is an elevation in section of one of the piezoelectric assemblies of the invention together with its mounting, a portion of the center of the assembly being omitted for purposes of clarity;

FIG. 3 is a cross-sectional view of the embodiment, taken in the direction of sight lines 3-3 of FIG. 2;

FIG. 4 is a cross-sectional view of the embodiment taken in the direction of sight lines 4-4 on FIG. 2;

FIGS. 5 and 6 show respectively a plan view and elevation of a central core assembly of piezoelectric elements;

FIGS. 7 and 8 show respectively a plan view and elevation of an outer cylindrical assembly of toroidally-shaped piezoelectric elements;

FIG. 9 is a schematic wiring diagram showing the electrical circuits for the respective top and bottom assemblies of piezoelectric elements;

FIG. 10 is a cross-sectional elevation of a pair of adjacent piezoelectric elements showing the method of joining them conductively; and

FIG. 11 is a plot of deflection as a function of driving frequency of the fatigue testing machine of my invention.

Throughout the drawings, similar reference characters indicate corresponding parts. Furthermore, dimensions of certain of the parts as shown in the drawings may have been modified and/or exaggerated for the purposes of 3 clarity of illustration and understanding of the invention.

Referring now to FIG. 1, there is shown an elevation of the general assembly of the apparatus of this invention.

A suitable base 2 is provided, supported on the legs 4 in conventional manner, and provided with a bottom shelf 6.

Mounted on base or support 2 is the testing apparatus of this invention generally indicated by numeral 8. It comprises two support portions and 12 either of square or circular shape, these portions being held apart by the spacer rods 14 having theaded ends 16 in conventional manner to provide spacing shoulders. Nuts 17 hold the upper and lower support portions on the rods 14. The separation of the support portions 10 and 12 provides a space within which a specimen 42 to be tested may be held as will be described below. Laterally extending clamps 18 are suitably fastened to the rods 14, and mounted at the ends of these clamps are the vibration absorbing mounts 20, the latter being suitably fastened to brackets 22 in a manner to isolate the vibrations of the assembly 8, the brackets 22 being securely fastened to the top of support 2.

In order to bring the central portion of the assembly 8 above the level of the base 2, the support 12 is shown as fitting freely within an opening 23 in base 2, portions of the base being broken away to show the support 12.

Mounted on the support 10 is an upper piezoelectric transducer assembly indicated generally by numeral 30, the details of which and how it is mounted on support 10 being given below. Suspended from the support 12 is a lower piezoelectric transducer assembly indicated generally by numeral 32, this assembly being mounted, as will be explained below, directly to the support 12. From the central cores of each of the assemblies 30, 32 there extend toward each other the chucks 34 and 36 into which may be screwed the specimen holders 38 and 40 be tween which may be clamped a specimen 42 for testing.

Mounted on the platform 6 is a motor 44 coupled to an oil pump 46. A reservoir of cooling oil 48 is mounted on platform 6, and an oil filler 50 is connected to the outlet of the pump 46. Oil from the reservoir 50 passes into the assembly 32 via the pipe 52 wherein it circulates around the piezoelectric elements therein, and then emerges from assembly 32 to pass via pipe 54 to assembly where similarly it circulates around the piezoelectric elements therein, then emerges and passes via pipe 56 to the oil reservoir 48 where, if desired, it may be cooled, and then passes via pipe 58 to the pump 46.

Suitable electric connections are made, as indicated above, to the assemblies of piezoelectric elements in the assemblies 30 and 32, so that when an alternating voltage is applied to the elements, the specimen holders 38- are moved away and toward each other by the piezoelectric elements, thus applying tensile and compressive forces to the specimen 42.

While the fatigue tester proper '8 is shown as being mounted on a support 2 with the oil pumps, filters and oil reservoirs being mounted on the platform 6, nevertheless, the details of mounting the tester itself on such a support are conventional, as well as the details of mounting the cooling oil means, and thus these may be changed without effecting the invention itself.

Referring now to FIGS. 2-4, the construction of the upper piezoelectric assembly 30' will now be described. The assembly comprises two stacks of piezoelectric elements, an inner core stack 60, cylindrical in shape which is mounted within an outer stack 62 the elements of which are in the form of rings or annuli. The material of the piezoelectric elements in all cases is preferably lead titanate-lead zirconate, this material being preferred because of its large motion per applied unit of voltage. However, if desired, other piezoelectric materials may be used where different characteristics of the fatigue tester itself may be desired.

Cir

Referring first to the inner core 60, and in particular to FIGS. 6 and 10, as previously indicated the piezoelectric elements 64 are stacked together in face to face relationship, and thus form a generally cylindrical column or stack. The elements are fastened together (see FIG. 10) by first electroplating the faces of the elements with a layer of silver 66. Thereafter, a layer of epoxy cement 68 is applied over the surfaces which have been electroplated with silver. A perforated nickel sheet is located between the elements as shown in FIG. 10. The perforations on the sheet '70 form protrusions in both directions from the plane of the sheet and these protrusions contact the silvered surfaces to provide electrical contact between the elements. In this way, the individual elements 64 are connected together, both mechanically and electrically. (In FIG. 10, the thicknesses of connective layers and of the nickel sheet 70 has been exaggerated in order to reveal the details of construction more clearly.)

At the upper end of the assembly or column 60 (see FIGS. 3, 5 and 6) the topmost piezoelectric element 65 is cemented to an electrically insulating disc 72 which may be, for example, a ceramic such as aluminum oxide (Al -O or fiberglass reinforced epoxy resin. Mounted on top of the disc 72 are four insulating bars or pieces 74 (also made of A1 0 ceramics, or of fiberglass reinforced epoxy resin) arranged at right angles to each other as shown in FIG. 3. Fitting in the quadrants formed by the bars 74 are the connector segments 76, 78, and 82 made of electrically conductive metal such as copper. Overlying the assembly of the segments 76-82 and the insulating cross 75 is another insulating disc 84 which, like disc 72, may be made of aluminum oxide A1 0 ceramic. The upper surface of the disc 84 is cemented to the bottom surface of. a metal mounting block 86 (best seen in FIG. 2). 1

It is to be noted that the several components 64, 72, 74, 7682, 84, and 86 are all firmly cemented together by using, for example, an epoxy cement. Thus, the entire cylindrical columnar assembly 60 is cemented to the metal block 86. At the lower end of the column 60, the bottom piezoelectric element 67 is cemented to an insulating disc 88 made, for example, A1 0 ceramic and similar to insulating disc 72. A piezoelectric element 90 (similar to elements 64 and preferably of the same material) has its two flat faces silver plated, and then the upper silver plated surface is cemented to the insulating disc 88. The lower silvered face of element 90 is similarly cemented (using epoxy cement) to the insulating disc 92 which also may be made, for example, of A1 0 ceramic.

The lower surface of the disc 92 is cemented with epoxy cement to the upper flat surface of the upper chuck 34, thus anchoring this upper chuck to the column 60' so that when the column expands or contracts, the chuck will move with it. The entire assembly 60, the metal mounting block 86 and the upper chuck 34 are tied together by a central tension rod 93 which extends through the entire stack. The tension rod is threaded at both ends as at 93a and 93b. Conventional nuts 94 and washers 95 are provided to engage the threaded ends of the tension rod. The bore in the stack 60 through which the tension rod extends is provided with an insulating tube 96 to insulate the metal rod from the piezoelectric stack 60.

The function of the rod 93 is to preload the entire stack assembly so that the cemented joints between the stack elements are always under compression. In this way, the possibilities of failure in the stack as the result of poor adhesion are substantially reduced.

The chuck 34 is of conventional structure, and is provided with a threaded bore 97 as shown to receive threaded therein the upper specimen holder 38. The chuck 34 is cylindrical and fits with a smooth sliding fit into a bore 98 which is provided in a metal mounting collar 99. Chuck 34 is grooved and fitted with a rubber O-ring 100 in order to provide a sliding, liquid-tight seal between the chuck and the bore 96.

The base of annular cylindrical column 62- is supported on the collar 99 as shown in FIG. 2. Each of the elements 102 of the stack 62 is made of the same material as the elements 64, and the opposing faces of each element are silver plated and then cemented together with an intermediate nickel sheet by epoxy cement as on the inner column.

The uppermost ring 103 of the column 62 has cemented on top thereof an insulating ring 104 made of a ceramic such as A1 Cemented to the top surface of the ring 104 is a metal ring or collar 106, the latter being attached by means of several machine screws 108 to the mounting block 86. Thus, the top end of column 60 is fastened to the top end of column 62, both of the columns being attached to the mounting block 86. The annular outer column 62 is also preloaded by the four tension rods as described in connection with the inner assembly 60 for the reasons previously stated. These have not been specifically illustrated in order to simplify the drawings.

The bottommost piezoelectric ring 109 is cemented to an insulating ring 110 of, for example, A1 0 of the column 62 and the latter is cemented to and surmounts four sectors 112, 114, 116 and 118 of an annulus, these sectors being made of a conductive metal such as brass or copper. In turn, the sectors 112-118 are cemented to the top surface of an insulating ring 120, which may be made, for example, of A1 0 The bottom surface of ring 120 is in turn cemented to a suitably provided flat surface within the cup-shaped collar 99.

Supported on the fiat upper surface of the rim 122 of the collar 99 is a metal ring 124 which is suitably fastened thereto by means of conventional screws :126. The ring 124 is supplied with an inner circumferential rabbet 128 into which is nested an O-ring 129. Ftting down into the ring 124 and welded to it in such manner that the O-ring 129 forms a liquid-tight seal against it is a casing 130, the upper end of the casing in turn being welded to a cap 132. The cap 132 is provided with an oil outlet 134 into which a suitably threaded compression fitting elbow indicated generally by numeral 136 may be fitted. Cap 132 is centrally provided with a threaded aperture 138 through which passes a cylindrical oil-inlet bushing 140, the latter being provided with a flange and a gasket 142 to provide an oil-tight seal against the underside of cap 132. The outer end of the bushing 140 is threaded as shown to receive nut $143 which clamps the bushing 140 in place. The inner end of the bushing 140 is provided with neck 144, the latter being a slidable fit in a bore 146 provided within the mounting block 86. A suitably provided O-ring 148 is held in a groove within the wall of bore 146 in order to provide an oil tight seal between the neck 144 and the bore.

It will be noted that the outer diameter of column 60 and the inner diameter of column 62 are so dimensioned as to leave an annular space 150 between the two columns. Mounting block 86 is provided with a pair of bores 152 and 154 which interconnect, and bore 154 interconnects with bore 146. The radial distance of the bore 152 from bore 146 is such as to have the opening of bore 152 overlie the annular space 150.

By this construction, oil may flow into the upper assembly 30 by means of bushing 140, bores .146, 154, and 152, down through the annular space 150. Then by means of the spaces 156 between sectors 112-118, the oil may flow into the angular space 158 between the outer wall of column 62 and the casing 130, the oil emerging via the oil outlet 134.

An inverted cup-shaped protective cover 160 is formed to fit down over the casing 130, and is then fastened in place by means of several conventional machine screws 161.

Mention has previously been made of the provision of an ambient temperature compensating support for one of the assemblies in order to eliminate any static loading on the test specimen due to, for example, thermal expansion or contraction of the several parts of the apparatus. This feature will now be described, and as shown in FIG. 2, it is provided in respect to the upper assembly 30.

The collar 98 is firmly mounted on top of a cylindrical piston member 164 having a central bore 168, attachment being made by conventional means such as the machine screws 166 which pass through suitably provided holes in the member 164 and are threaded into the collar 98. The central bore 168 of the piston is made just slightly larger than the outer diameter of the chuck 34, and preferably the chuck is tapered as shown in FIG. 2 so as to be sure the chuck will have complete clearance from the bore. Piston 164 is provided with a circumferential, radiallyextending ring portion 170 having a cylindrically-shaped peripheral face 171 the latter being provided with a peripheral groove 172. The piston is provided with the internally connecting bores 174 and 176, the latter opening at the bottom surface of the piston and being provided with a threaded enlargement 178 for an oil compression fitting; and bore 174 terminating in the bottom of groove 172.

The support portion 10 is in the form of a heavy walled ring, the inner wall 180 of which is threaded as shown in FIG. 2. Threaded into the ring or base 10 is a cup-shaped annulus 182 having a bore 184, and being shouldered to provide bore 186 larger in diameter than bore 184. The diameter of bore 186 is such as to receive the peripheral ring portion 170 with a sliding fit, the diameter of peripheral ring 170 being a few thousandths less than the diam eter of bore 186. The diameter of bore 184 is such as to receive the lower portion 163 of piston 162 with a smooth sliding fit, said body portion being provided with a groove to hold an O-ring 188 in order to provide a slidable liquidtight seal.

Mounted on top of the annulus 182 is an annulus 190 having an inner diameter which fits over the upper portion 164 of piston 162 with a smooth sliding fit. An O-ring 192 suitably held in a groove in the said portion 164 makes a slidable liquid-tight seal with the collar 190. Collar 190 is fastened securely to the annulus 182 in conventional manner, as by means of the machine screws 194. If desired, and as shown, an O-ring 196 is suitably held in a groove in the bottom surface of annulus 190 in order to make a liquid-tight seal between the latter and the top of the annulus 182.

In use, glycerin is pumped into the groove 172 under pressure via bores 174 and 176, as a result of which the glycerin will blow upwardly and downwardly between the outer surface of the peripheral ring 170 and the inner wall 186 to fill the spaces 198 and 200 above and below the annular ring 170. The reason that glycerin is specifically mentioned is because it has the highest bulk modulus of the common fluids. (If desired, a heavy oil may be used instead.) Because of the slow leak of the glycerin past the spacing between the annulus 170 and the bore 186, the piston 162 is enabled to move up and down very slowly over a long period of time. If, for example, the separator rods 14 of the assembly should elongate due to a rise in their temperature, ordinarily such elongation would put a static loading on the test specimen 42. However, a rise in temperature of this 'kind generally takes place over a relatively long period of time, and thus the same period of time will give piston 162 an opportunity to shift within the surrounding casing structure comprising the elements 182 and 190, which are, of course, fastened into the upper portion 10. Thus, the support for the upper piezoelectric assembly is compensated against temperature changes of a slow-occurring kind. However, in view of the bulk modulus of the glycerin or heavy oil in the spaces 198 and 200, and the small clearance between the peripheral surface of the ring 170 and the inner wall 186, the piston is (to all intents and purposes) rigidly mounted within the upper sub-base 10 insofar as high frequencies are concerned. Similarly, if an initial static loading of mechanical nature is put upon the test specimen during a loading operation, this static loading will be relieved by a shift of the piston 162.

The structure of the bottom assembly 32 is the same as that of the upper assembly 30 insofar as the stacks of piezoelectric elements are concerned, the means of cementing them together, and their attachment to the several metal supporting parts, etc. There is, however, one major difference and that is that no static load compensating means is provided for the bottom assembly since only one such compensation is necessary to cancel out static loadings. As a result of this, the piston structure 162 is not used, and instead the collar support 98 of assembly 30 is duplicated for assembly 32 but is attached firmly to the support 12 conventionally by the use of machine screws. The support 12 is provided with a central bore 168 analogous to bore 168 in piston 162, and as in the upper assembly, the chuck 36 of the lower assembly is so dimensioned as to be without hindrance in its passage through the bore 168.

Referring to FIG. 6, which is an elevation of the core assembly 60, the elements 64 are shown in their stacked arrangement, with electrical connections between the elements being made by means of the connecting straps 202 and 204. Strap 202 is bodily insulated from the elements 64 themselves by means of the insulating strip 206, and in similar manner connecting strap 204 is bodily insulated from the elements 64 by means of insulating strip 208. The upper end of strap 202 is connected to quadrant 80 (see FIG. 3) by means of a connecting screw 210 which passes through the strap and into the quadrant 80. Strap 202 is also suitably connected (as by means of a lead wire running from the strap through the insulator 206) to the joint 212 between the element 65 and the insulator disc 72. (That is, strap 202 is connected to the top surface of element 65.) Strap 202 is similarly connected to the top surface of every other element 64. Since the top surface of one element is already electrically connected to the bottom surface of the element lying on top thereof, the strap 202 is at the same time connected to the bottom surface of alternate elements, all these connections being made at connecting points 214, 216, etc.

In similar fashion, the upper end of strap 204 is connected to the quadrant 76 by means of the terminal screw 218 which passes through the insulating strip 208. Thereafter, connecting strap 204 makes its first connection with the bottom of element 65 and the top of the next lower element 64, and thereafter to the bottom and top of alternate pairs of elements.

In diametrically opposite apertures 220 and 222 in connecting ring 106 are inserted a pair of insulating bushings 224 and 226. Through these insulators pass terminal screws 228 and 230 respectively to screw into suitably provided holes 229 and 231 respectively in quadrants 76 and 80 respectively.

Referring now to FIGS. 3, 4 and 8, the connections to the annular assembly 62 are as follows: an electrical connection strap 236 overlies an insulating strip 238 vertically along one side of the assembly, and diametrically opposite on the other side lie the electrical connecting strap 240 and the insulating strip 242. In this instance, the connecting strap 236 is connected to the bottom face of element 103 and the upper face of adjacent element 102, the point of this connection on strap 236 being indicated by numeral 244. In similar fashion, connections 246, 248, 250, etc. are made to alternate adjacent sets of faces of the elements 102. Similarly, connecting strap 240 is connected to the top surface of element 103 as indicated at numeral 252, and then is connected to alternate adjacent sets of faces as by connections 254, 256, 258, etc.

The lower end of connecting strap 236 is connected directly to quadrant 116 at the bottom of the assembly 62, and the lower end of connecting strap 240 is similarly connected directly to quadrant 112, these connections be ing made by the respective terminal screws 260 and 262 which are threaded into the quadrants. At the top of the two columns 60 and 62 (see FIG. 3), a short connecting strap 264 connects connecting strap 240 to the terminal screw 228. In similar fashion, diametrically opposite on the assembly, the short connecting strap 266 connects the connecting strap 236 to the terminal screw 230.

Through suitably provided holes in the upstanding rim of mounting collar 98 pass electrically insulating bushings 268, these bushings having inserted therethrough the power lead terminal screws 270 (which is threaded into quadrant 116) and 272 (which is threaded into quadrant 112).

Thus, upon a source of electrical voltage being applied to the power leads 270 and 272, it is connected by the above described connecting strapping to the several elements of the inner core and the annuli or rings of the outer column in such manner that these elements are all in parallel and are so connected that if upon a given polarity of voltage applied to terminal screws 270 and 272 the assembly 60 contracts, simultaneously the assembly 62 expands.

The element 90 is a piezoelectric measuring element by means of which the excursion of the chuck 34 is measured and thus the amounts of strain and stress which are imposed on the test specimen. The material of element 90 is preferably the same as the material of the elements 64 and 102. Connections are made to the measuring crystal 90 as follows: Referring to FIG. 6, a connecting strap 278 extends vertically up the length of the column assembly 62, and overlies the strip of electrically insulating material 280. The strap 278 is connected to the bottom silvered face of the element 90 by the conventional solder connection 282, and the top of the strap is connected to the quadrant 78 at the upper end of the column 60 by means of the terminal screw 284 which is suitably threaded into the quadrant. Through a suitably provided hole in the collar 106 at the top of column 62 passes an electrically insulating bushing 286, and through the latter passes the terminal screw 288 which is threaded into quadrant 78. A short strap 290 connects terminal screw 288 with a connecting strap 290 which extends down the annular column 62 and terminates at the terminal screw 292 which is threaded into the quadrant 114, the connecting strap 290 overlying an insulator strap 294 in its extension along column 62.

Through another suitably provided hole in the collar 98 passes an electrically insulating bushing 296 through which passes the terminal screw 298 which is threaded into quadrant 114, terminal 298 being one of the external connections to element 90.

In similar manner, on a diametrically opposite side of the column 60, a similar conductive strap 300 is provided, this connecting strap extending upwardly along the column 60 to the top thereof, overlying an electrically insulating strip 302 as it does so. The lower end of connecting strap 300 is attached by a conventional solder joint 304 to the upper silvered face of element 90, and the upper end of strap 300 is connected to the quadrant 82 by means of the terminal screw 306 which threads into the quadrant.

In a suitably provided hole in the collar 106 passes an electrically insulating bushing 308 through which passes a terminal screw 310 which is also threaded into the quadrant 82. A short connecting strap 312 connects the terminal 310 with a connecting strap 314 which extends down the outside of column 62 in a manner similar to connecting strap 290, strap 314 being insulated from the column by an electrically insulating strip 316 underlying strap 314. The lower end of strap 314 is connected to quadrant 118 by means of terminal screw 318 which screws into the quadrant. Through a suitably provided hole in collar 98, there passes an insulating bushing 320 through which passes the terminal screw 322 which is threaded into quadrant 118, terminal 322 being the other external connection to the element 90.

With the above connections, it is clear that any voltage induced across the element because of forces on the chucks in the machine which hold the test specimen will be led to the terminal screws 298 and 322, at which point the voltage may be impressed on a suitable measuring instrument in order to calibrate the machine in a desired manner.

In a manner exactly like that described for the connections of the arrays 60 and 62, and the measuring element 90, connections are made for an identically dimensioned and shaped pair of arrays 60 and 62 and a measuring crystal 90', in the lower assembly 32. Since, in view of the above description, it will be clear to the person skilled in the art how to make such an assembly and electrical connections, detailed drawings of these connections and further description thereof are not given herein.

Referring now to FIG. 9, there is shown a schematic wiring diagram of the above described apparatus. A source of alternating potential 324 is shown, the frequency of this power source being the frequency at which it is desired to test the specimen 42. In actual practice, it has been found that a frequency of to 600 cycles per second is a frequency in which fatigue testing of the specimen may be done at an accelerated rate very rapidly. However, since it will be desirable to go above and below this range, it is suggested that the power source 324 be capable of being adjusted to deliver frequencies higher and lower than the range given, at the required voltage. It is also of course possible to supply signals having complex random waveforms from a generator such as generator 324. Indeed the ability of the apparatus of my invention to respond to such waveforms is one of its most important features. The voltage source 324 is connected by leads 326 and 328 to the lead wires 330 and 332. Lead wires 330 and 332 connect to terminals 270 and 272 for stacks 60 and 62 of piezoelectric elements, and to like terminals 270 and 272' for stacks 60' and 62' of piezoelectric elements. As indicated above, faces of elements 102 of column 62 are connected together by lead 240, the latter connecting to terminal screw 272. Via the strap 264 and quadrant 76, lead 240 is also connected to strap 204 which connects faces of the elements 64 of stack 60. In similar manner, the other faces of the elements 102 of stack 62 are connected together by lead 236, the latter connecting to terminal 270. Via the connecting strap 266 and quadrant 80, lead 236 is also connected to lead 202 which connects the other faces of the elements 64 of stack 60.

In respect to the piezoelectric elements of assembly 32, faces of elements 102' of column 62 are connected together by lead 240', the latter connecting to terminal screw 272'. Via the strap 264' and quadrant 76', lead 240 is also connected to strap 204' which connects faces of the elements 64 of stack 60'. In similar manner, the other faces of elements 102' of stack 62' are connected together by lead 236, the latter connecting to terminal 270. Via the connecting strap 266' the quadrant 80', lead 236' is also connected to lead 202 which connects the other faces of the elements 64' of the stack 60.

Prior to assembly, each of the individual piezoelectric elements is polarized by the application of a direct voltage in a manner well known in the art. Assuming, as polarized, that one of the faces of a piezoelectric element may be designated as plus and the opposite face as minus, then when the elements are assembled to form the column or stacks 60,60, 62, 62' all of the surfaces of the elements of a given stack designated as plus are cemented together. It is then clear that when the various elements are connected as described above, and a voltage of a given polarity is applied to, for example, the surface designated as plus, the elements will contract, and when the reverse polarity is applied, then the elements expand.

Referring to FIG. 2, which is representative of the upper assembly of piezoelectric elements 64 and 102, these respectively making the inner core assembly 60 and the outer annulus or cylindrical assembly 62, the connections made to the various interfaces of the respective elements in each of the inner and outer columnar assemblies is such that when the polarity of the applied voltage is such 10 as to cause each of the elements 64 to contract, that same polarity is connected to the elements 102 so as to cause them to expand. The result is that with this stated polarity of voltage, the upper chuck 34 is pulled upwardly.

The connections and wiring to the assembly of core and outer cylindrical column of piezoelectric units in the lower assembly 32 is the same as found in the upper assembly 30, with the result that when said stated polarity of voltage is applied to give a pulling upward of the chuck 34, the proper polarity is applied to the elements of the lower assembly 32 to pull the chuck 36 downwardly. As a result, the specimen 42 being tested is placed under considerable tensile force.

When the polarity of the supply voltage 324 shifts to the opposite phase, then the respective elements will so react as to move the chucks 34 and 36 toward each other thus relieving the tensile force on the specimen 42 and, if desired, placing the specimen 42 in compression.

Of course, if desired, the upper assembly 30 with its static loading compensator may be used alone, with the separators 14 being mounted firmly to the table 2, thus eliminating the lower assembly 32 which operates the specimen chuck 36 and specimen holder 40. As a result of this, the specimen 42 will thus experience only one-half of the tensile stretch from the assembly 30 as will be experienced when both of the assemblies 30 and 32 are used. As described immediately above, the machine would not be symmetrical and would, therefore, have a less favorable vibratory respond (the lowest natural frequency would be important.) Thus, the preferred form is shown and described in this application.

Referring now to FIG. 11, there' is shown a graph of a typical response of an apparatus constructed in accordance with the above teachings. In the graph, the deflection of the apparatus is represented as mils (0.001 inch), and the frequency applied to the assembly is given in cycles per second. It will be noted that the response of the particular elements used shows a resonance peak at approximately 900 cycles, and since it is not desired to operate the machine at this point, the machine is operated within the frequency range of approximately 50 cycles to 650 cycles. In actual practice, the voltage applied to the stacks from an alternator (324) was 2,000 A.C., this corresponding to a field strength of 8,000 volts per inch for each element since the elements are approximately A inch thick.

This gives a safe margin under the depolarizing voltage of about 10,000 volts per inch for the particular elements used. (The voltages given above are RMS values.) In the graph, the deflection is that across the specimen itself as a function of driving frequency on the piezoelectric elements. The forces engendered across the specimen are approximately 2,000 pounds.

In apparatus following the above teaching which was actually constructed and tested, the diameter of the elements of the inner cores, that is, the elements 64 and 64' was approximately two inches and their thickness was approximately inch. The elements of the cylindrical assemblies 62 and 62', that is, the annuli 102 and 102' had an outside diameter of approximately 3% inches and a thickness of approximately A inch. The central core assemblies 60 and 60' contained 43 elements acting as the specimen stressing elements, and similarly the outer cylindrical stack 62 and 62' each contained 41 piezoelectric elements. The measuring elements and 20 were approximately two inches in diameter and approximately A inch in thickness.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

It is to be understood that the invention is not limited in its application to the details of construction and arrangemen of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways.

1 1 Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

In particular, it is to be understood that while I have described the apparatus of my invention in terms of deflection in the translational mode, it is possible to utilize the described construction for torsional operation. For torsional mode fatigue testing the stack construction is as described but uses crystals whose deflection in response to applied excitation is movement in torsion as opposed to axial movement. Additionally, in a fatigue testing machine operating in the torsion mode the static load elimination described above is not required.

As many changes could be made in the above description, without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense, and it is also intended that the appended claims shall cover all such equivalent variations as come within the true spirit and scope of the invention.

Having described my invention what I claim as new and desire to secure by Letters Patent is:

1. Apparatus for testing the strength of a material comprising:

a base having first and second support portions, the

latter being spaced apart to provide a specimenreceiving space therebetween;

first piezoelectric material means mounted on the first support portion,

first clamping means operatively attached to said first piezoelectric means for movement thereby and being adapted to grip one end of said specimen and hold the latter within said space;

second piezoelectric material means mounted on the second support portion so that its direction of motion upon being electrically excited is opposed to the direction of said first piezoelectric means, and

second clam-ping means operatively attached to said second piezoelectric means and adapted to grip the other end of said specimen in opposition to said first clamping means; whereby said specimen may be stressed in response to force applied thereto by said piezoelectric means.

2. The apparatus of claim *1 in which said first and second piezoelectric means comprise respectively first and second sets of at least two columns of piezoelectric material, each column being adapted when excited electrically to expand and contract axially; one end of a first of the columns of the first set being mounted on said first support portion; one end of a second of the columns of the first set having attached thereto said first clamping means; one end of a first of the columns of the second set being mounted on the second support portion; one end of a second of the columns of the second set having attached thereto said second clamping means; the other ends of said first and second columns of each set being operatively attached together.

3. The combination defined in claim 2 in which the piezoelectric materials of the clumns are so connected electrically that upon the imposition of an electrical voltage of a given polarity on the material, one of the columns of each set expands and the other column of the same set contracts, said sets being so positioned with respect to each other and being electrically interconnected so that upon electrical excitation of the columns with an electrical signal of alternating polarity the clamping means moves toward and away from each other.

4. The apparatus of claim 3 in which one of said columns of each of said sets is a first stack of polarized annuli of piezoelectric material, the faces of one polarity of the annuli being conductively connected in parallel, and faces of the opposite polarity being connected in parallel; the other of said columns of each of said sets is a second stack of polarized plates of piezoelectric material, faces of one polarity of the plates being conductively connected in parallel, and faces of the opposite polarity being connected in parallel; the second stack being enclosed by the first stack; and an electrical connection between the faces of a given polarity of the annuli in each set and the faces of opposite polarity of the plates in the same set.

5. The apparatus of claim 4 including an electrical connection between the faces of one polarity of the annuli of one set and the faces of like polarity of the annuli of the second set; and an electrical connection between the faces of opposite polarity of the annuli of said one set and the faces of like polarity of the annuli of said second set.

6. Apparatus for testing the strength of a material comprising:

a base having first and second support portions, the

latter being spaced apart to provide a specimenreceiving space therebetween;

first piezoelectric material means mounted on the first support portion, and being positioned thereon so that the direction of its expansion and contraction upon being electrically excited is along a line traversing said space;

first clamping means operatively attached to said first piezoelectric means for movement thereby and being adapted to grip one end of said specimen and hold the latter within said space;

second piezoelectric material means mounted on the second support portion so that its motion upon being electrically excited is opposed to the direction of said first piezoelectric means,

second clamping means operatively attached to said second piezoelectric means and adapted to grip the other end of said specimen in opposition to said first clamping means; and

static load compensating means supporting at least one of said clamping means for compensating for any forces, other than those due to said piezoelectric means, which would otherwise stress said specimen in a direction along said line.

7. The apparatus of claim 6 in which said compensating means comprises two movable members movable with respect to each other and having portions closely adjacent but not touching whereby to provide a bleed-space between the members, the bleed-space being filled with a viscous fluid.

8. The apparatus of claim 7 in which one of said movable members is a piston and the other of said movable members is a cylinder therefor, the diameter of the piston being less than the internal diameter of the cylinder whereby to provide said bleed-space between the piston periphery and the cylinder internal wall.

9. The apparatus of claim 8 in which one of the movable members is attached to the piezoelectric means at a place remote from said first clamping means, and the other of the movable members is attached to the second support portion, whereby to provide a viscous mounting for the first clamping means in relation to the second clamping means.

10. An axially expansible and contractible assembly of piezoelectric material elements comprising an outer stack.

and an inner stack of said elements; the elements of the outer stack being annuli and polarized, the faces of like polarity of said annuli being cemented together adjacent each other with the annuli lying on a common axis; the elements of the inner stack being plates and polarized, the faces of like polarity of said plates being cemented together adjacent each other wtih the approximate centers of the plates lying on said common axis; an end of the outer stack being mechanically connected to an end of the inner stack; a first electrical connection connecting in parallel the faces of said annuli of a given porality; a

second electrical connection connecting in parallel the faces of said annuli of polarity opposite to said given polarity; a third electrical connection connecting in parallel all faces of said plates of said given polarity; a fourth electrical connection connecting in parallel all faces of said plates of polarity opposite to said given polarity; a fifth electrical connection connecting said first and fourth con nections; and a sixth electrical connection connecting said second and third connections.

References Cited UNITED STATES PATENTS 2,936,612 5/1960 Mason 7391X 3,062,044 11/1962 Jonson et a1. 739l JERRY W. MYRACLE, Primary Examiner US. Cl. X.R. 

